Modern nuclear reactors produce highly radioactive fission products and actinide elements which must be prevented from entering the biosphere over periods ranging from 10 to 1,000,000 years. The current policy and practice is to convert these high-level nuclear wastes to solid forms, such as glasses or ceramics, which are then encapsulated in metal containers and buried underground in impervious, stable rock formations.
There are a great many patents directed to the structure of containers for nuclear wastes. Most contain only brief disclosures of the materials from which the containers are made. Stainless steel is named repeatedly, as well as iron, steel, lead, concrete, steel lined with copper, brass, zirconium alloy, cadmium, tantalum, tungsten, mercury, molybdenum, and sandwich constructions employing various gels and fibers between layers of metal. Perhaps the closest to the subject invention is U.S. Pat. No. 3,659,107, issued to Seele et al. on Apr. 25, 1972, which describes a radioactive fuel capsule, not a waste container, but which states that it may be made of various refractory materials, including nickel and alloys thereof.
Because of the presence of chromium in stainless steel and other components in the other container materials now in use, they are all more or less thermodynamically unstable in the geochemical environments of natural rock formations, and it is accepted that they can become corroded and decompose within a few tens of years after burial. Accordingly, primary emphasis in immobilizing nuclear wastes is placed upon the insolubility of the radioactive elements in the solidified waste and on the impermeability and ion-exchange properties of the rock medium. However, while this solution has been the best available, it is far from completely satisfactory and it has long been obvious that, if the integrity of the metal container itself could be guaranteed for periods exceeding a million years, the problems associated with safe storage of nuclear wastes would be substantially reduced.